(1) Field of the Invention
The present invention relates to a novel chemical adsorbate compound chemically adsorbed onto a surface of a substrate material. This chemical adsorbate compound is usable as a functional film for modifying properties of the surface of the substrate material, and as a liquid crystal alignment film for controlling the alignment of liquid crystal molecules. The present invention also relates to a liquid crystal alignment film employing this novel chemical adsorbate compound. The present invention further relates to a liquid crystal display device comprising this liquid crystal alignment film as a component thereof.
(2) Description of the Prior Art
In recent years, as the size and weight reduction of information processing equipment advances, liquid crystal displays have increasingly found widespread use. The liquid crystal display devices generally require liquid crystal alignment films for aligning liquid crystal molecules, but not many materials are suitable for candidate materials for the liquid crystal alignment films. As the performance of the liquid crystal devices becomes increasingly higher, there is an increasing demand for novel materials for liquid crystal alignment films which exhibit improved characteristics.
Conventionally, polymers such as polyimides and polyvinyl alcohols have been utilized for the materials for liquid crystal alignment films. However, such films made of polymers are generally large in thickness and electrically insulative, and therefore tend to cause image sticking, degradation of effective electric field, and similar undesirable effects. Moreover, such films are fixed to substrates by so-called anchoring effect, thus showing weak bonding or adhesive characteristics.
In addition, such films made of polyimides or the like have a structure such that polymer molecules are bonded each other in an intricately intertwined manner, and liquid crystal molecules cannot enter inside the films. Therefore, in such films, only the surface layer where the foremost ends of the polymer molecules protrude serves to control the alignment of liquid crystal molecules, and the rest of the portion of the film does not influence the alignment of liquid crystal molecules. Hence, the films made of polymers cannot exhibit sufficient alignment control performance over liquid crystal molecules.
Furthermore, while the films made of polymers are generally given alignment control characteristic by rubbing, alignment system made by rubbing is susceptible to external degrading factors such as heat or friction. Thus, the liquid crystal alignment films made by rubbing have a drawback that the films cannot exhibit sufficient alignment control characteristic or alignment stability. Further, rubbing causes the following drawbacks.
i) When a film surface has roughness, depressed portions of the film surface cannot be rubbed. The unrubbed portions cause alignment defects, which in turn cause image sticking and unevenness in displayed images.
ii) Rubbing treatment causes static charge, which damages the TFTs formed underneath the alignment film.
iii) By rubbing, dust is produced from a rubbing cloth (cotton cloth or the like) and film surface. The dust can cause uneveness in displayed images and undesirable variation in the cell gap.
In order to provide a solution to such problems of rubbing as described above, some non-contact alignment techniques have been suggested. However, none of the techniques has yet achieved completely satisfactory results. For example, Japanese Unexamined Patent Publication No. 5-53118 discloses the following technique; a layer of a photosensitive compound is formed on a substrate, and grooves having a prescribed pattern are formed in the photosensitive compound layer by light exposure and heat treatment, thus realizing an alignment control characteristic by the grooves. This technique, however, requires a large light energy for producing the grooves. Moreover, uniform grooves are difficult to form, and uneveness in displayed images tends to occur. The alignment control performance is also insufficient.
Japanese Unexamined Patent Publication No. 7-72483 discloses the following technique; alignment control characteristic in an alignment film is realized by applying a linear polarized light to a layer of a compound for forming the alignment film containing polyimide or polyimide precursor to polymerize the polyimide or polyimide precursor. However, this technique employs polyimide, which is an organic polymer, and therefore cannot solve the problem of the large film thickness, which induces an increase in liquid crystal driving voltage. Another problem is that the adhesiveness of the resultant alignment film to the substrate is not sufficient.
Japanese Unexamined Patent Publication No. 7-318942 disclose the following technique; in an alignment film, a molecular structure having an alignment control characteristic is realized by applying a light from an oblique direction to the alignment film having a polymer structure to create different bonds or decomposition in the molecular chains in the film. However, this technique also employs alignment films made of organic polymers such as polyimide, polyvinyl alcohol, polystyrene, and the like. Therefore, this technique as well cannot solve the above-described problems of large film thickness and small adhesiveness to the substrate. In addition, this technique requires that a light be applied from an oblique direction to the alignment film in order to produce a pretilt angle, and this leads to increase in manufacturing cost since an accurate light irradiation from an oblique direction calls for a high accuracy apparatus for light irradiation.
Further, twisted-nematic mode or the like liquid crystal display devices employing prior art liquid crystal alignment film have another problem of narrow viewing angles. In view of this problem, for example, Japanese Unexamined Patent Publication No. 6-173135 discloses the following technique; an alignment film comprising a plurality of micro-regions having different liquid crystal alignment directions are realized by repeating the consecutive steps in which an alignment film is rubbed in a certain direction, thereafter prescribed regions of the rubbed film are coated with a resist, and the film is rubbed in the reverse direction. However, in order to form a plurality of domains having different alignment directions by a rubbing method (contact method), it is necessary to repeat complicated steps of masking and rubbing each time corresponding to the divided domains. Accordingly, by this technique, production efficiency is degraded, and the problems of the static charge and dust produced by rubbing are further worsened.
In order to improve viewing angles, it is. possible to employ the non-contact methods as described above. However, those prior art non-contact techniques (Japanese Unexamined Patent Publication Nos. 5-53118, 7-72483, and 7-318942) have the problems of large film thickness and insufficient adhesiveness to the substrate, and therefore cannot provide satisfactory liquid crystal alignment films.
In view of these problems, the present inventors have proposed a technique for producing a novel nanosized liquid crystal alignment film in Japanese Unexamined Patent Publication No. 3-7913. In this technique, an alignment film is composed of a monomolecular layer in which a silane-based chemical adsorbate compound (also referred to as a xe2x80x9csurfactantxe2x80x9d) is chemically adsorbed onto a substrate surface. According to this technique, an ultra thin transparent film firmly bonded to the substrate surface can be easily formed in an efficient manner. Furthermore, this technique can provide, without rubbing, a liquid crystal alignment film having a certain alignment control performance. However, in this technique, there still is room for improvement in the thermal stability and alignment control performance of the alignment film.
The present invention comprises a series of groups, namely, a first group, a second group, a third group, and a fourth group. The series of the invention is intended to provide solutions to the above-described problems and drawbacks in prior art at one time.
Accordingly, it is a first object of the present invention to provide a novel chemical adsorbate compound capable of easily producing functional films for modifying surfaces of substrate materials and liquid crystal alignment films for controlling the alignment of liquid crystal molecules. More specifically, the first object of the invention is to provide a compound such that; a) the compound is capable of forming a nanosized film by being chemically adsorbed onto a surface of a substrate material, and b) the compound is colorless and stable to a light in the visible range (400 to 700 nm wavelength range), whereas reactive to a light in the ultraviolet range (200 to 400 nm wavelength range), and thereby the molecules are crosslinked with each other by being irradiated with ultraviolet light, forming a stable film structure. The present invention also provides a method of producing such a compound in an efficient manner.
It is a second object of the invention is to realize, by using the above chemical adsorbate compound, a non-rubbed liquid crystal alignment film having an excellent alignment control performance and thermal stability.
It is a third object of the invention to realize, by employing the above liquid crystal alignment film, a liquid crystal display device capable of achieving a wide viewing angle and clear images.
Now, each group of the invention will be described below.
(1) The First Group
i) A chemical adsorbate compound in accordance with the first group of the invention was accomplished based on the view that a chalcone skeleton is highly photosensitive. In accordance with the first group of the invention, there is provided a chemical adsorbate compound represented by the following chemical formula 101. The chalcone skeleton means a skeleton represented by the chemical formula 104. 
(In the formula, n is an integer from 1 to 20, and X is a halogen.) 
The compound shown by the chemical formula 101 has xe2x80x94Oxe2x80x94SiX3 group serving as a chemically adsorbed group, and therefore can be easily adsorbed onto a surface of a substrate material having hydrophilic groups (for example, OH groups, COOH groups, NH2 groups, NH groups, SH groups, etc.). Thus, a monomolecular film having an extremely small thickness can be easily formed on the surface of a substrate material. The compound having a chemical structure represented by the chemical formula 101 is colorless, transparent, and chemically stable in the visible light range.
In addition, the compound of the chemical formula 101 has a carbon-carbon double bond in the molecular structure, and the ultraviolet absorption peak of the compound is in a longer wavelength range than that of conventional compounds of this type. Accordingly, by employing a light having such a wavelength, the adsorbed molecules can be crosslinked with each other without causing side reactions (decomposition). More specifically, the compound of the chemical formula 101 according to the invention has a carbonyl group at the 4xe2x80x2 position of the benzene ring. The carbonyl group can be conjugated with the chalcone skeleton, and the conjugate length is thereby made longer than that in the case where the compound is made only of the chalcons skeleton. Therefore, the electrons in the conjugation are delocalized, and the energy level is reduced. As a result, the ultraviolet absorption wavelength is shifted to a longer wavelength range.
If the ultraviolet light employed has a short wavelength in the case where the main reaction by the light irradiation is crosslinking or polymerization, decomposition tends to occur as a side reaction. However, since the compound represented by the chemical formula 101 has the absorption peak in a longer wavelength range in the ultraviolet range, it is possible to effectively excite the carbon-carbon double bonds in the chalcone skeletons and thereby to crosslink the adsorbed molecules each other via the bonds without causing side reactions. The film made by such crosslinking has excellent water repellency and durability.
In addition, the compound represented by the chemical formula 101 has a hydrocarbon group where n=1 to 20. When n in the hydrocarbon group is in the foregoing range, the molecular length is appropriate, and therefore the compound can exhibit excellent molecular alignment characteristic and perform smooth crosslinking.
Accordingly, the compound represented by the chemical formula 101 can form an ultra thin monomolecular film firmly chemically bonded to a substrate material surface, and the resultant film shows excellent properties such as good water repellency, durability, chemical stability, and weather resistance. Hence, such a film is remarkably useful as a functional film for modifying the surface of a substrate material, and the excellent properties are particularly useful in forming a liquid crystal alignment film. The reasons why the film is suitable for a liquid crystal alignment film will now be discussed below.
As has already mentioned above, the chemical adsorbate compound of the chemical formula 101 is easily chemically bonded to the substrate material surface. Therefore, when the solution containing the chemical adsorbate compound of the chemical formula 101 contacts with the surface of the substrate as a component of a liquid crystal cell, one end of each adsorbed molecule (the end adjacent to the xe2x80x94Oxe2x80x94Si bond group) is chemically bonded to the substrate while the other end protrudes in a direction far from the substrate surface. Thereby, it is made possible to form a thin film composed of an aggregate of a multiplicity of the adsorbed molecules arrayed along the substrate surface.
In such a thin film, composed of the aggregate of the adsorbed molecules, liquid crystal molecules can enter the gap spaces between the adsorbed molecules. The inclination (pretilt angle) and alignment orientation (pretilt orientation) of the liquid crystal molecules slotted in the gap spaces with respect to the substrate are restricted by the inclination and/or alignment orientation (hereinafter these are also collectively referred to as an xe2x80x9calignment directionxe2x80x9d) of the adsorbed molecules with respect to the substrate. Thus, by controlling the alignment direction of the adsorbed molecules, the alignment direction of the liquid crystal molecules can be arbitrarily controlled.
By employing the above-described chemical adsorbate compound, it is possible to form a film having an advantageous structure such that every individual adsorbed molecule has an influence on the alignment control of liquid crystal molecules, and the resultant film therefore exhibits a remarkably high efficiency in the alignment control performance in relation to the film thickness. In addition, the resultant film has a remarkably small thickness, leading to an excellent light transmission characteristic. Moreover, since the film is not made of polymer, the adverse effects as an electrical resistive/insulative film are small, and the film does not hinder the electric field for driving liquid crystals. Furthermore, since the adsorbed molecules in the thin film are firmly bonded to the substrate by chemical bond, the film does not easily come off from the substrate surface.
Thus, a liquid crystal alignment film capable of increasing the brightness and contrast ratio and reducing the driving voltage can be realized by employing the above-described chemical adsorbate compound. In addition, in the film employing the chemical adsorbate compound, after chemically adsorbed to the substrate, the adsorbed molecules can be crosslinked with each other at the carbon-carbon double bonds by applying an ultraviolet light to the film surface. As a result, the liquid crystal alignment characteristic of the resultant film does not degrade by external degrading factors such as heat, contacts with water or organic solvents, and so forth. Furthermore, when a polarized light is used in the ultraviolet light irradiation, crosslinking of the molecules can be caused along a specific direction. Thus, by specifying the direction of polarization, a desired alignment control characteristic can be obtained in the liquid crystal alignment film.
It is considered that crosslinking of the molecules serves to stabilize the liquid crystal alignment control characteristic since the configuration of the molecules each other becomes fixed by crosslinking of the adsorbed molecules with each other. By contrast, conventional liquid crystal alignment films (such as the above-mentioned polymer films composed of polyimides) have such a structure that long main chains are densely intertwined each other, only the surface portion of the films can contribute to the alignment control over liquid crystals, Consequently, conventional alignment films cannot achieve a sufficient alignment control performance. Moreover, if external degrading factors such as heat or friction are inflicted on conventional rubbed alignment films, the alignment control performance of the films is changed or degraded. Further, polymer films such as polyimide films are large in the film thickness and electrically insulative, and therefore degrade the effective electric field for driving liquid crystals and cause image sticking.
ii) The above-described chemical adsorbate compound may be such that n is an integer from 5 to 10, and X is chlorine in the chemical formula 101.
If n in the chemical formula 101 is less than 5, in other words, if the length of the hydrocarbon group is short, the angle or proportion of the chalcone skeleton arrayed vertically with respect to the substrate material becomes small, thereby decreasing the efficiency in the crosslinking reaction. In order for the crosslinking to progress at high efficiency, the relative positions of the carbon-carbon double bonds of the adsorbed molecules to be crosslinked are important, i.e., the photosensitive sites must be in contact with or adjacent to each other. However, when many of the molecules are arrayed horizontally with respect to the substrate, the photosensitive sites do not easily come in contact with or close to each other.
On the other hand, if n is greater than 10, the degree of freedom of molecules with respect to the substrate material surface becomes too large, which also leads to the disadvantage that the photosensitive sites do not easily come in contact with or close to each other and the efficiency of the crosslinking decreases. Accordingly, the value n is preferable to be in the range of 5 to 10.
Chloride is preferable as X in the chemical formula 101. When the X is chloride, the chemical adsorbate compound can be easily chemisorbed onto the substrate material by dehydrochlorination, and in addition, the adsorbed molecules per se can be easily synthesized.
iii) The chemical adsorbate compound in accordance with the first group of the invention can be produced by the following method comprising at least the steps of: reacting benzaldehyde and 4-acetylbenzoic acid by aldol condensation to synthesize a first chalcone derivative represented by the chemical formula 102:
the first chalcone derivative wherein a carbonyl group is bonded at the 4xe2x80x2 position of the benzene ring in the chalcone skeleton; and after the step of reacting benzaldehyde and 4-acetylbenzoic, reacting an alcohol derived from the first chalcone derivative with an SiX4 where X is a halogen in an inert gas atmosphere by dehydrohalogenation, to synthesize a second chalcone derivative having an xe2x80x94Oxe2x80x94SiX3 group and a characteristic group represented by the chemical formula 103:
By employing this method, it is made possible to easily and efficiently produce the second chalcone derivative in which the carbonyl group is bonded at the 4xe2x80x2 position of the benzene ring in the chalcone skeleton, and the characteristic group having the xe2x80x94Oxe2x80x94SiX3 group is bonded at the side of the carbonyl group. In the second chalcone derivative produced by this method, the carbon-carbon double bond (xe2x80x94CHxe2x95x90CHxe2x80x94) is highly sensitive to ultraviolet light, and the SiX3 has high chemical adsorbing performance. In addition, since the main axis of the molecule is linear, a desirable alignment control of liquid crystal molecules is possible.
iv) The above-described method for producing a chemical adsorbate compound according to the first group of the invention may be such that the second chalcone derivative is a compound represented by the chemical formula 101:
(In the formula, n is an integer from 1 to 20, and X is a halogen.)
In the chemical adsorbate compound composed of the compound represented by the chemical formula 101, the carbon-carbon double bond in the chalcone skeleton is highly photosensitive, and the chemically adsorbed molecule has an appropriate length, which results in a desirable alignment control performance. In addition, the compound is easy to synthesize.
v) The above-described method of producing a chemical adsorbate compound may be such that n in the chemical formula 101 is in the range of 5 to 10, and X is chlorine.
Accordingly, the same advantageous effects as described in the forgoing ii) can be achieved.
Further accounts of the first group of the invention are given below. The chemical adsorbate compound in accordance with the first group of the invention was accomplished based on the view that the chalcone skeleton is highly photosensitive. The chemical adsorbate compound is characterized in that a carbonyl group is bonded at the 4xe2x80x2 position of the benzene ring in the chalcone skeleton, a divalent functional group (preferably a linear hydrocarbon group) having an appropriate length is bonded to the carbonyl group, and an xe2x80x94Oxe2x80x94SiX group (where X is a halogen) is bonded to one end of the divalent functional group.
The value n of the hydrocarbon group (CH2)n in the chemical formula 101 should be an integer from 1 to 20, preferably in the range of 3 to 16, more preferably in the range of 5 to 10. The reason is, as already stated above, the efficiency of the crosslinking is degraded when n is either too large or too small. Accordingly, a preferred embodiment of the first group of the invention includes, for example, a compound represented by the following chemical formula 105:
In the chemical adsorbate compound in accordance with the first group of the invention, other divalent functional groups may be employed in place of (CH2)n in the chemical formula 101. Examples of such divalent functional groups include a divalent functional group containing a carbon-carbon double bond or a carbon-carbon triple bond in part of the hydrocarbon groups, a divalent functional group in which hydrogen in the hydrocarbon groups is substituted by other functional groups (e.g., methyl groups, methyl halide groups, hydroxyl groups, cyano groups, or the like) and/or atoms (e.g., F, Cl, Br, I, or the like), and a divalent functional group in which a Cxe2x80x94Oxe2x80x94C (ether) bond or a Cxe2x80x94COxe2x80x94Cxe2x80x94 (carbonyl) bond is substituted for a Cxe2x80x94C bond in the hydrocarbon group.
The above-described chemical adsorbate compound in accordance with the first group of the invention can be produced by a method having a step of reacting benzaldehyde with 4-acetylbenzoic acid by aldol condensation. The detail of this reaction will be given in Examples to be described hereinbelow. When the method in which benzaldehyde and 4-acetylbenzoic acid are used as starting materials is employed, a desired compound can be synthesized with a good reaction efficiency. Nevertheless, the chemical adsorbate compound can be synthesized by other producing methods, and, for example, it is possible to employ such a method that, using chalcone as a starting material, a characteristic group is added thereto.
(2) The Second Group
The second group of the invention relates to a liquid crystal alignment film and a liquid crystal display device utilizing the alignment film.
i) In accordance with the second group of the invention, there is provided a liquid crystal alignment film comprising an aggregate of adsorbed molecules chemically adsorbed by siloxane bonds directly or via a layer of another substance onto a substrate having an electrode formed thereon, and each of the adsorbed molecules comprising an xe2x80x94Oxe2x80x94Si bond group at an end of the molecule and a characteristic group represented by the chemical formula 201:
The liquid crystal alignment film of the above-described configuration is composed of an aggregate of adsorbed molecules in which one end of each adsorbed molecule (the end adjacent to the xe2x80x94Oxe2x80x94Si bond group side) chemically bonds to the substrate while the other end protrudes in a direction far from the substrate surface. In the aggregate of adsorbed molecules which contain the characteristic groups of the chemical formula 201 and the xe2x80x94Oxe2x80x94Si bond groups, since each individual adsorbed molecule is firmly bonded to the substrate by chemical bond, the film does not easily come off from the substrate, In addition, liquid crystal molecules can enter the gap spaces between the adsorbed molecules in the aggregate, and the inclination (pretilt angle) and alignment orientation (pretilt orientation) of the liquid crystal molecules with respect to the substrate are restricted by the inclination and/or alignment orientation (hereinafter these are also collectively referred to as an xe2x80x9calignment directionxe2x80x9d) of the adsorbed molecules with respect to the substrate.
Accordingly, by employing the above-described configuration of the invention, it is made possible to form a liquid crystal alignment film composed of a film in which every adsorbed molecule has an influence over the alignment control of liquid crystal molecules. Such an alignment film exhibits a remarkably high efficiency in the alignment control performance in relation to the film thickness. In addition, the resultant film has a remarkably small thickness, leading to an excellent light transmission characteristic and durability. Moreover, this film does not cause much undesirable effect as an electrically resistive film, and therefore does not hinder the electric field for driving liquid crystals. Thus, by employing the above-described configuration of the invention, a liquid crystal alignment film capable of increasing the brightness and contrast ratios and reducing the driving voltage can be realized.
An example of the adsorbed molecules comprising an xe2x80x94Oxe2x80x94Si bond group and a characteristic group represented by the foregoing chemical formula 201 is a compound represented by the following chemical formula 202.
(In the formula, n is an integer from 1 to 20.)
The adsorbed molecules represented by the chemical formula 202 have an appropriate molecular length, which leads to a good alignment control over liquid crystal molecules. In addition, the resultant alignment film has excellent durability since the molecules can be firmly bonded to the substrate via Si and moreover the molecules each other can be firmly bonded via Si.
It is noted that the adsorbed molecules need to have an appropriate molecular length in order to obtain a good liquid crystal alignment performance and crosslinking performance, and accordingly, n in the hydrocarbon group (CH2)n is preferable to be in the range of 1 to 20. Nevertheless, in terms of crosslinking performance, n is preferable to be in the range of 3 to 16, and more preferable to be in the range of 5 to 10. In order for the crosslinking to be performed with high efficiency, the relative positions of the carbon-carbon double bonds of the adsorbed molecules to be crosslinked are important, i.e., the photosensitive groups must be in contact with or adjacent to each other. However, when n is small (or example, less than 5), the angle or proportion of the adsorbed molecules arrayed vertically with respect to the substrate becomes small. In other words, many of the molecules are arrayed horizontally with respect to the substrate, and this reduces the degree of contacts at carbon-carbon double bonds (photosensitive sites), which in turn reduces the efficiency in crosslinking.
On the other hand, if n is large, (for example, greater than 10), the degree of freedom of molecules with respect to the substrate material surface becomes too large, which reduces the degree of contacts at the photosensitive sites in the adsorbed molecules each other. Thus, the efficiency of the crosslinking also decreases in this case, and consequently, the resultant film shows a small crosslinking rate.
In the aggregate of adsorbed molecules that constitutes the liquid crystal alignment film, the adsorbed molecules may be aligned in a predetermined direction. Thereby, a uniform liquid crystal alignment performance is obtained.
In the above-described alignment film, and an inclination and/or alignment direction of long axes of the adsorbed molecules with respect to a substrate surface plane may be different in a plurality of divided domains next to each other. The plurality of divided domains refers to a plurality of micro-domains such that a single pixel region is divided into a plurality of domains in a pattern-like manner. When such a so-called multi-domain type liquid crystal alignment film, in which the alignment direction of the adsorbed molecules is controlled in each of the micro-domains, is employed, a viewing angle dependency in displayed images is reduced since the transmitted light in each pixel is made up of a plurality of bundles of lights.
In the above-described alignment film, the adsorbed molecules may be crosslinked with each other at the carbon Cxe2x80x2 and/or the carbon Cxe2x80x3 in the chemical formula 201 or 202. In the aggregate of adsorbed molecules, the inclination and alignment direction of the adsorbed molecules are not varied by external degrading factors such as heat or friction since the adsorbed molecules are firmly bonded each other by crosslinking. Accordingly, a highly reliable liquid crystal alignment film can be obtained.
In the above-described liquid crystal alignment film, a film thickness thereof may be from 0.5 nm to less than 10 nm. Within this thickness range, high alignment control efficiency relative to film thicknesses is achieved, and no hindrance to light transmission and electric field is caused. Thus, the utility of the film as a liquid crystal alignment film is further increased.
In the above-described liquid crystal alignment film, the liquid crystal alignment film may be a monomolecular thin film. When the film is a monomolecular thin film, the efficiency in liquid crystal alignment control remarkably increases since every individual adsorbed molecule can directly influence the alignment of liquid crystal molecules.
Next, detailed below are methods for producing liquid crystal alignment films in accordance with the second group of the invention. In one embodiment of the second group of the invention, there is provided a method for producing a liquid crystal alignment film comprising at least the steps of preparing a chemisorption solution by dissolving in a non-aqueous solvent a chemical adsorbate compound having an xe2x80x94Oxe2x80x94Si bond group at a molecular end and a characteristic group represented by the chemical formula 203:
and contacting the chemisorption solution with a substrate surface having a pixel electrode formed thereon to adsorb molecules of the chemical adsorbate compound in the chemisorption solution onto the substrate surface by siloxane bonds.
The above-described method may further comprise a step of drain-drying including, after the step of contacting the chemisorption solution, rinsing the substrate surface having the adsorbed molecules thereon with a nonaqueous solvent for cleaning, and draining and drying the solvent for cleaning remaining on the substrate surface in a predetermined direction. In addition, the above-described method may further comprise, after the step of drain-drying, irradiating the adsorbed molecules on the substrate surface with a polarized ultraviolet light so that the adsorbed molecules are crosslinked with each other at a carbon-carbon double bond shown in the chemical formula 203.
Further, the above-described method may be such that the steps of drain-drying and irradiating with a polarized ultraviolet light are repeated a plurality of times, the direction of the draining and drying is varied each time of the step of drain-drying, and one of a) a region to be irradiated with the ultraviolet light and a direction of the ultraviolet light, b) a region to be irradiated with the ultraviolet light and an incident angle of the ultraviolet light, and c) a region to be irradiated with the ultraviolet light, a direction of the ultraviolet light, and an angle of applying the ultraviolet light, is varied each time of the step of irradiating, whereby an inclination and/or alignment direction of long axes of the adsorbed molecules is/are varied in a plurality of domains such that a single pixel region is divided into the plurality of domains in a pattern-like manner. Accordingly, a multi-domain type liquid crystal alignment film can be efficiently fabricated with a high productivity.
Further in the above-described method, an aprotic solvent may be used for the nonaqueous solvent for cleaning in the step of rinsing, whereby unreacted chemical adsorbate compound is removed by rinsing the substrate surface to form a monomolecular thin film.
Further in the above-described method, a mixed solvent of an aprotic solvent and a protic solvent may be used for the nonaqueous solvent for cleaning in the step of rinsing, whereby unreacted chemical adsorbate compound is removed by rinsing the substrate surface to form a monomolecular thin film. The mixed solvent of an aprotic solvent and a protic solvent is preferable in that the capability of dissolving the chemical adsorbate compound and the evaporation rate can be appropriately controlled.
The significance of the above-described methods is now discussed below. When the substrate is contacted with the solution of the chemical adsorbate compound having the characteristic group represented by the chemical formula 203 and the xe2x80x94Oxe2x80x94Si bond group at a molecular end, there is formed a thin film in which the chemical adsorbate compound is bonded to hydrophilic groups on the substrate surface by siloxane bonds (this is also referred to as xe2x80x9cchemical adsorptionxe2x80x9d). The thin film is composed of an aggregate of adsorbed molecules aligned in such a manner that one longitudinal end (the end adjacent to the xe2x80x94Oxe2x80x94Si bond group) of each adsorbed molecule is chemically bonded to the substrate while the other end is extended towards a direction far from the substrate surface. In such a thin film composed of the aggregate of the adsorbed molecules, liquid crystal molecules can enter the gap spaces between the adsorbed molecules. The inclination (pretilt angle) and alignment orientation (pretilt orientation) of the liquid crystal molecules slotted in the gap spaces with respect to the substrate are restricted by the inclination and/or alignment orientation (hereafter these are also collectively referred to as an xe2x80x9calignment directionxe2x80x9d) of the adsorbed molecules with respect to the substrate. Thus, by specifying the alignment direction of the adsorbed molecules, the alignment direction of the liquid crystal molecules can be controlled.
In the visible light range, the compound having the characteristic group represented by the chemical formula 203 is colorless, transparent, and chemically stable, while the carbon-carbon double bond thereof is highly sensitive to ultraviolet light. Accordingly, by irradiating the substrate with ultraviolet light, the adsorbed molecules can be crosslinked with each other via the carbon-carbon double bonds after the chemical adsorbate compound is chemically adsorbed onto the substrate. When a polarized ultraviolet light is used in this step, the direction of crosslinking can be controlled to a certain direction corresponding to the direction of polarization of the polarized ultraviolet light. By the polarized light irradiation, the adsorbed molecules on the substrate surface are realigned. By this realignment, the molecules each other are crosslinked, and consequently, the resultant alignment film is not easily changed by external degrading factors such as heat or friction.
In the above-described method, the chemical adsorbate compound having an xe2x80x94Oxe2x80x94Si bond group and a characteristic group represented by the foregoing chemical formula 203 may be a compound represented by the following chemical formula 204.
(In the formula, n is an integer from 1 to 20, and X is a halogen.)
The compound represented by the chemical formula 204 can be easily and firmly adsorbed onto the substrate surface, and the photosensitivity of the carbon-carbon double bond is high. The resultant thin film formed by the compound being chemically adsorbed exhibits excellent transparency and durability. Moreover, the compound has an appropriate molecular length for controlling the alignment of liquid crystal molecules. Hence, by employing the compound of the chemical formula 204 in the above-described method, a desirable liquid crystal alignment film can be produced with high productivity.
Some characteristic points in the above-described methods are further detailed below. In the step of drain-drying according to the methods of the invention, first, by rinsing, unadsorbed chemical adsorbate compound excessively existing on the substrate surface is removed therefrom, and thereafter, by draining and drying, the solution for cleaning is removed. By these procedures, a monomolecular thin film can be formed such that the adsorbed molecules are aligned in a direction of drain-drying. The alignment state of the adsorbed molecules thus obtained by the drain-drying can be varied by repeating the drain-drying, and is susceptible to external degrading factors (such as heat or friction). Accordingly, the alignment state thus obtained is referred to as a xe2x80x9cpre-alignmentxe2x80x9d herein.
It is noted that examples for the methods for the draining and drying include a method by pulling up the substrate being soaked in the cleaning solution at a predetermined angle, and a method by blowing an air current onto the substrate surface from a predetermined direction.
In the above-described step of irradiating, the thin film surface (aggregate of the adsorbed molecules) is irradiated with a polarized ultraviolet light. Since the adsorbed molecules comprising the characteristic group represented by the chemical formula 203 are highly photosensitive, the molecules are reacted and crosslinked with each other at the carbon-carbon double bonds in a certain direction corresponding to the direction of polarization. The direction of polarization may be the same direction as the direction of the pre-alignment described above, or may be a different direction therefrom. In either case, by applying a polarized light, the adsorbed molecules can be realigned in a certain direction corresponding to the direction of polarization. It is, however, preferable that the direction of drain-drying and the direction of polarization not be crossed at 90xc2x0, but be staggered a little, preferably by several degrees or more. This is because, if the directions are crossed at 90xc2x0, the molecules are randomly aligned in two directions.
While the mechanism is not entirely understood, it has been confirmed by experiments that by pre-aligning the adsorbed molecules and thereafter irradiating with polarized ultraviolet light, the crosslinking is smoothly performed in a certain direction and the effect of the alignment treatment by the polarized ultraviolet irradiation is enhanced.
It is to be noted that the alignment by the polarized ultraviolet light irradiation is referred to as a xe2x80x9crealignmentxe2x80x9d herein, in order to distinguish it from the pre-alignment described above. It is also to be noted that the compound molecules after being chemically adsorbed onto the substrate are referred to as xe2x80x9cadsorbed molecules,xe2x80x9d and the compound before being adsorbed are referred to as xe2x80x9cchemical adsorbate compound.xe2x80x9d In addition, it has been confirmed by experiments that the thickness of the thin film in which the adsorbed molecules are chemically adsorbed onto the substrate surface is approximately the molecular length of the adsorbed molecule (in the order of nanometers).
The differences between the liquid crystal alignment films according to the present invention and conventional liquid crystal alignment films are as follows. In the conventional liquid crystal alignment films (for example, polymer films composed of polyimides mentioned above), the long main chains are densely intertwined each other, and consequently, only the surface portion of the films can contribute to the alignment control over liquid crystals. For this reason, the conventional alignment films cannot achieve a sufficient alignment control performance. Moreover, in conventional alignment films in which the alignment control characteristic is obtained by rubbing, the alignment control characteristic is changed or degraded by external degrading factors such as hear or friction. Furthermore, polymer films composed of such as polyimides can hinder the light transmission and driving of liquid crystal since such films have a large film thickness and high electrical resistivity.
Nonetheless, even when the liquid crystal alignment film is made of a monomolecular thin film, the alignment stability can be insufficient if the adsorbed molecules are not crosslinked with each other. For example, the foregoing chemical adsorbate compound disclosed in Japanese Unexamined Patent Publication No. 3-7913 does not have a photoreactive group, and therefore is not capable of chemically linking the adsorbed molecules each other. Therefore, in the liquid crystal alignment films employing this chemical ad sorbate compound, the alignment control characteristic tends to be degraded by heat at about 200xc2x0 C.
Next, liquid crystal display devices according to the second group utilizing the above-described alignment films are described below.
In one embodiment of the second group of the invention, there is provided a liquid crystal display device comprising a pair of opposed substrates, a liquid crystal alignment film provided on a surface of at least one of the substrates, the surface having an electrode thereon, and a liquid crystal enclosed in a cell gap between the pair of substrates, the liquid crystal display device wherein the liquid crystal alignment film comprises an aggregate of adsorbed molecules chemically adsorbed directly or via a layer of another substance onto the surface of the substrate having an electrode thereon by siloxane bonds, and each of the adsorbed molecules comprises an xe2x80x94Oxe2x80x94Si bond group at an end of the molecule and a characteristic group represented by the chemical formula 201:
In the above-described liquid crystal display device, a pretilt angle and/or pretilt orientation of liquid crystal molecules in the cell gap may be controlled by an inclination and/or alignment direction of long axes of the adsorbed molecules with respect to a substrate surface plane.
In addition, the above-described liquid crystal display device may be such that the liquid crystal alignment film has a plurality of domains such that a single pixel region is divided into the plurality of domains in a pattern-like manner, and an inclination and/or alignment direction of long axes of the adsorbed molecules with respect to a substrate surface plane is/are different in the plurality of domains next to each other.
In another embodiment of the second group, there is provided an in-plane switching type liquid crystal display device comprising a substrate, a pixel electrode provided on the substrate, a counter electrode also provided on the substrate on which the pixel electrode is provided, and a liquid crystal alignment film formed on a surface of the substrates on which both of the electrodes are provided; the liquid crystal display device wherein the liquid crystal alignment film comprises an aggregate of adsorbed molecules chemically adsorbed directly or via a layer of another substance onto the surface of the substrate having the electrode thereon by siloxane bonds, and each of the adsorbed molecules comprises an xe2x80x94Oxe2x80x94Si bond group at an end of the molecule and a characteristic group represented by the foregoing chemical formula 201.
In the above-described liquid crystal display device (hereafter including the in-plane switching type where applicable), a pretilt angle and/or pretilt orientation of liquid crystal molecules in the cell gap may be controlled by an inclination and/or alignment direction of long axes of the adsorbed molecules with respect to a substrate surface plane.
In the above-described liquid crystal display device, the adsorbed molecules may be crosslinked with each other at the carbon Cxe2x80x2 and/or the carbon Cxe2x80x3 in the chemical formula 201.
In the above-described liquid crystal display device, each of the adsorbed molecules may be composed of a compound represented by the chemical formula 202:
(In the formula, n is an integer from 1 to 20.).
In the above-described liquid crystal display device, a film thickness of the liquid crystal alignment film may be from 0.5 nm to less than 10 nm.
In the above-described liquid crystal display device, the liquid crystal alignment film may be a monomolecular thin film.
It is considered that in an ideal monomolecular layer, each single constituent molecule is arrayed along a substrate surface and no molecules are overlaid thereon. However, in practice, it is not so easy to form a perfect monomolecular layer, and moreover, a monomolecular layer that is not so perfect can sufficiently accomplish the objects of the invention. Accordingly, it is to be understood that a monomolecular thin film according to the present invention may be such a thin film that it is recognized as a substantially monomolecular layer. For example, it may be such a film partially having a layer of a plurality of molecules in which unadsorbed molecules are overlaid on the adsorbed molecules onto a substrate. Also included is such a film partially having a layer of a plurality of molecules in which molecules that are themselves unbonded to the substrate but bonded to the molecules already fixed to the substrate, or further other molecules are bonded to the unbonded molecules. It is to be understood that monomolecular thin films according to the present invention includes these films partially having a layer of a plurality of molecules.
In addition, although the above discussion has been made based on the premise that an aggregate of adsorbed molecules is composed only of one type of chemical adsorbate compound, it is to be understood that other adsorbate compounds may be mixed with the adsorbed molecules of the present invention.
(3) The Third Group
i) In accordance with the third group of the invention, there is provided a chemical adsorbate compound represented by the following chemical formula 301. It is noted that a chalcone skeleton means a skeleton represented by the following chemical formula 304.
(In the formula, n is an integer from 1 to 20, and X is a halogen.) 
The compound having a chemical structure shown by the chemical formula 301, xe2x80x94Oxe2x80x94SiX3 serves as a chemically adsorbed group. Therefore, the compound can be easily adsorbed onto a surface of a substrate material having hydrophilic groups (for example, OH groups, COOH groups, NH2 groups, NH groups, SH groups, etc.). Thus, a monomolecular film having an extremely small thickness can be easily formed on the substrate material surface. The compound having a chemical structure represented by the chemical formula 301 is colorless, transparent, and chemically stable in the visible light range.
In addition, the compound of the chemical formula 301 has a carbon-carbon double bond in the molecular structure, and a linear hydrocarbon group is ether-linked to the 4xe2x80x2 position of the benzene ring in the chalcone skeleton in the compound. In the compound having such a chemical structure, the ether-linked linear hydrocarbon group serves to increase the density of the conjugated electrons delocalized and stabilized in the chalcone skeleton. The compound is thereby further stabilized, and the ultraviolet absorption wavelength derived from the electrons is further shifted towards a longer wavelength range. As a result, the compound has an absorption peak in the longer wavelength range. Thus, by irradiating the compound with an ultraviolet light having a wavelength in the long wavelength range near the visible range, the electrons at the carbon-carbon double bond in the chalcone skeleton are excited and activated, and the adsorbed molecules can be crosslinked with each other via the bonds. Here, if a long wavelength ultraviolet light is employed, a good film can be obtained since such an ultraviolet light does not easily cause a side reaction (decomposition). The resultant film exhibits excellent durability and abrasion resistance, as well as excellent chemical stability, transparency, and water repellency.
From the above discussion, it will be understood that the compound represented by the chemical formula 301 is useful as a functional film for modifying the surface of a substrate material, and particularly useful in forming a liquid crystal alignment film. The reasons why the film formed of the compound is suitable for a liquid crystal alignment film will now be discussed below.
The chemical adsorbate compound of the chemical formula 301 is easily chemically bonded to the substrate material surface. Therefore, when the solution containing the chemical adsorbate compound of the chemical formula 301 (chemisorption solution) contacts with the surface of the substrate as a component of a liquid crystal cell, one end of each adsorbed molecule (the end adjacent to the xe2x80x94Oxe2x80x94Si bond group) chemically is bonded to the substrate while the other end protrudes in a direction far from the substrate surface. Thereby, it is made possible to form a thin film composed of an aggregate of a multiplicity of the adsorbed molecules arrayed along the substrate surface. In such a thin film composed of the aggregate of the adsorbed molecules, liquid crystal molecules can enter the gap spaces between the adsorbed molecules. The inclination (pretilt angle) and alignment orientation (pretilt orientation) of the liquid crystal molecules slotted in the gap spaces with respect to the substrate are controlled by the inclination and/or alignment orientation (hereinafter these are also collectively referred to as an xe2x80x9calignment directionxe2x80x9d) of the adsorbed molecules with respect to the substrate. Thus, by controlling the alignment direction of the adsorbed molecules, the alignment direction of the liquid crystal molecules can be arbitrarily controlled.
In addition, the resultant film is a monomolecular film having an extremely small thickness and therefore shows excellent light transmission characteristic. Further, the resultant film exhibits a remarkably high efficiency in the alignment control performance since every individual adsorbed molecules has an influence on the alignment control over liquid crystal molecules. Moreover, since the film is not made of polymer, the adverse effects as an electrical resistive/insulative film are small, and the film does not hinder the electric field for driving liquid crystals. Furthermore, since the adsorbed molecules in the thin film are firmly bonded to the substrate by chemical bond, the film does not easily come off from the substrate surface.
By applying a polarized ultraviolet light to the above-described film, the adsorbed molecules can be crosslinked with each other in a certain direction at the carbon-carbon double bonds. Thereby, a liquid crystal alignment film having a desired alignment control characteristic can be attained without rubbing. The resultant alignment film is chemically stabilized and therefore the alignment control characteristic is not varied by external degrading factors such as heat or friction. Hence, an advantageous non-rubbed liquid crystal alignment film can be formed by employing the above chemical adsorbate compound, and a liquid crystal display device capable of driving liquid crystal at a low voltage and having an excellent brightness and contrast ratio can be realized by utilizing the liquid crystal alignment film.
It is considered that crosslinking of the molecules serves to stabilize the liquid crystal alignment control characteristic since the configuration of the molecules each other becomes stabilized by the crosslinking of the adsorbed molecules with each other. By contrast, in conventional rubbed liquid crystal alignment films, the alignment control changes by external factors such as heat or friction, which is apparent from the fact that repeating the rubbing causes a change in the characteristic. Also, since conventional liquid crystal alignment films made of polymers such as polyimides have such a structure that long main chains are densely intertwined each other, and only the surface portion of the films can contribute to the alignment control over liquid crystals. Consequently, conventional alignment films cannot achieve a sufficient alignment control performance. Further, conventional alignment films do not have a sufficient light transmission characteristic since they have large film thicknesses, and still further, they have a large electrical insulating characteristic, which necessitates a higher driving voltage.
ii) In the foregoing chemical adsorbate compound represented by the chemical formula 301, n may be an integer from 5 to 10 and X may be chlorine in the chemical formula 301.
If n in the chemical formula 301 is less than 5, in other words, if the length of the hydrocarbon group is shorter than 5 C, the efficiency in the crosslinking reaction decreases. In order for the crosslinking to progress at high efficiency, the relative positions of the carbon-carbon double bonds of the adsorbed molecules to be crosslinked are important, i.e., the photosensitive sites of the adsorbed molecules must be in contact with or adjacent to each other. However, when the length of the hydrocarbon group is short, the angle of the chalcone skeletons arrayed vertically with respect to the substrate material becomes small and the number of the molecules arrayed vertically also becomes small. Thus, many of the molecules are arrayed horizontally with respect to the substrate material, and therefore the photosensitive sites do not easily come in contact with or close to each other.
On the other hand, if n is greater than 10, the degree of freedom of molecules with respect to the substrate material surface becomes too large, which also leads to the disadvantage that the photosensitive sites do not easily come in contact with or close to each other and the efficiency of the crosslinking decreases. Accordingly, it is preferable that n=5 to 10.
The X in the chemical formula 301 is preferable to be chloride. When the X is chloride, the chemical adsorbate compound can be easily chemisorbed onto the substrate material by dehydrochlorination, and in addition, the adsorbed molecules per se can be easily synthesized.
iii) In another embodiment of the third group of the invention, there is provided a method for producing a chemical adsorbate compound for forming a thin film comprising at least the steps of reacting 4-hydroxybenzaldehyde and acetophenone by aldol condensation to synthesize a first compound having an hydroxyl group at the 4 position of the benzene ring in the chalcone skeleton, the first compound represented by the chemical formula 302:
and, after the step of reacting 4-hydroxybenzaldehyde and acetophenone, reacting an alcohol derived from the first compound with an SiX4 where X is a halogen in an inert gas atmosphere by dehydrohalogenation, to synthesize a second compound having at least an xe2x80x94Oxe2x80x94SiX3 group and a characteristic group represented by the chemical formula 303:
By employing this method, it is made possible to easily and efficiently produce the second compound in which the linear hydrocarbon group is ether-linked at the 4 position in the benzene ring in the chalcone skeleton and the xe2x80x94SiX3 group is ether-linked to the hydrocarbon group. In the second compound produced by this method, the carbon-carbon double bond (xe2x80x94CHxe2x95x90CHxe2x80x94) in the chalcone skeleton is highly sensitive to ultraviolet light, and the SiX3 has high chemical adsorbing performance.
iv) In the above-described method for producing a chemical adsorbate compound for forming a thin film, the second compound may be a compound represented by the following chemical formula 301:
(In the formula, n is an integer from 1 to 20, and X is a halogen.)
By employing the chemical adsorbate compound composed of the compound represented by the chemical formula 301, the carbon-carbon double bond in the chalcone skeleton becomes more highly photosensitive, and the ultraviolet light absorption peak is further shifted to a longer wavelength range. Furthermore, the main axis of the molecule is linear and the molecular length is appropriate, which results in a desirable alignment control. In addition, the compound is easy to synthesize.
v) In the above-described method for producing a chemical adsorbate compound, n may be an integer from 5 to 10, and X may be chlorine in the foregoing chemical formula 301. The advantageous effects by this are the same as described above.
Further accounts of the chemical adsorbate compound in accordance with the third group of the invention are given below. The chemical adsorbate compound of the third group of the invention was accomplished based on the view that the chalcone skeleton is highly photosensitive. The chemical adsorbate compound is characterized in that, as shown in the foregoing chemical formula 301, a linear hydrocarbon group is ether-linked at the 4 position of the benzene ring in the chalcone skeleton, and an xe2x80x94Oxe2x80x94SiX group (where X is a halogen) is attached thereto.
The value n of the hydrocarbon group (CH2)n in the chemical formula 301 should be an integer from 1 to 20, preferably in the range of 3 to 16, more preferably in the range of 5 to 10. The reason is, as already stated above, the efficiency of the crosslinking is degraded when n is either too large or too small. Accordingly, a preferred embodiment of the third group of the invention includes, for example, a compound represented by the following chemical formula 305;
The above-described chemical adsorbate compound in accordance with the third group of the invention can be produced by a method having a step of reacting 4-hydroxybenzaldehyde and acetophenone by aldol condensation so as to bond a hydroxyl group to the 4 position of the benzene ring in the chalcone skeleton. The detail of this reaction will be given in Examples to be described hereinbelow. When the method comprising at least the steps of reacting 4-hydroxybenzaldehyde and acetophenone by aldol condensation, and reacting an alcohol having a chalcone skeleton group with an SiX4 where X is a halogen in an inert gas atmosphere by condensation to synthesize a compound having an xe2x80x94Oxe2x80x94SiX4 bond, a desired compound can be synthesized with a good reaction efficiency. Nevertheless, the chemical adsorbate compound can be synthesized by other producing methods, and, for example, it is possible to employ such a method that, using chalcone as a starting material, a characteristic group is added thereto.
In the chemical adsorbate compound in accordance with the third group of the invention, other divalent functional groups may be employed in place of (CH2)n in the chemical formula 301. Examples of such divalent functional groups include a divalent functional group containing a carbon-carbon double bond or a carbon-carbon triple bond in part of the hydrocarbon groups, a divalent functional group in which hydrogen in the hydrocarbon groups is substituted by other functional groups (e.g., methyl groups, methyl halide groups, hydroxyl groups, cyano groups, or the like) and/or atoms (e.g., F, Cl, Br, I, or the like), and a divalent functional group in which a Cxe2x80x94Oxe2x80x94C (ether) bond or a Cxe2x80x94COxe2x80x94Cxe2x80x94 (carbonyl) bond is substituted for a Cxe2x80x94C bond in the hydrocarbon group.
(4) The Fourth Group
The foregoing and other objects of the present invention are accomplished, in accordance with the fourth group of the invention, by the provision of a liquid crystal alignment film comprising an aggregate of adsorbed molecules chemically adsorbed directly or via a layer of another substance onto a substrate having an electrode formed thereon, and each of the adsorbed molecules comprising an Si bond group at an end of the molecule and a characteristic group represented by the chemical formula 401:
The liquid crystal alignment film of the above-described configuration is composed of an aggregate of adsorbed molecules in which one end of each adsorbed molecule (the end adjacent to the Si bond group) chemically bonds to the substrate while the other end protrudes in a direction far from the substrate surface. In the thin film composed of such an aggregate of the adsorbed molecules, since each individual adsorbed molecule is firmly bonded to the substrate by chemical bond, the film does not easily come off from the substrate. In addition, liquid crystal molecules can enter the gap spaces between the adsorbed molecules in the aggregate, and the inclination (pretilt angle) and alignment orientation (pretilt orientation) of the liquid crystal molecules with respect to the substrate are restricted by the inclination and/or alignment orientation (hereinafter these are also collectively referred to as an xe2x80x9calignment directionxe2x80x9d) of the adsorbed molecules with respect to the substrate. Accordingly, by employing the above-described configuration of the invention, it is made possible to form a liquid crystal alignment film composed of a film in which every adsorbed molecule has an influence on the alignment control of liquid crystal molecules. Such an alignment film exhibits a remarkably high efficiency in the alignment control performance in relation to the film thickness. In addition, the resultant film has a remarkably small thickness, leading to an excellent light transmission characteristic and durability. Moreover, this film does not cause much undesirable effect as an electrically resistive film, and therefore does not hinder the electric field for driving liquid crystals.
Thus, by employing the above-described configuration of the invention, it is made possible to form a liquid crystal alignment film capable of improving the display characteristics of a liquid crystal display such as a brightness and contrast ratio, and reducing the driving voltage.
An example of the adsorbed molecules comprising anxe2x80x94Si bond group and a characteristic group represented by the foregoing chemical formula 401 is a compound represented by the following chemical formula 402.
(In the formula, n is an integer from 1 to 20.)
The adsorbed molecules represented by the chemical formula 402 have an appropriate molecular length because of the linear hydrocarbon group, which leads to a good alignment control over liquid crystal molecules. In addition, the resultant alignment film has excellent durability and adhesiveness to the substrate since the molecules are firmly bonded to the substrate via Si and moreover the molecules each other are also bonded via the remaining Si sites,
It is noted that the adsorbed molecules need to have an appropriate molecular length in order to obtain a good liquid crystal alignment performance and crosslinking performance. In order for the crosslinking to be performed with high efficiency, the relative positions of the carbon-carbon double bonds of the adsorbed molecules to be crosslinked are important, i.e., the photosensitive groups must be in contact with or adjacent to each other. However, when n is small (for example, less than 5), the angle or proportion of the adsorbed molecules arrayed vertically with respect to the substrate becomes small. In other words, many of the molecules are arrayed horizontally with respect to the substrate, and this reduces the degree of contacts at carbon-carbon double bonds (photosensitive sites), which in turn reduces the efficiency in crosslinking.
On the other hand, if n is large, (for example, greater than 10), the degree of freedom of molecules with respect to the substrate material surface becomes too large, which reduces the degree of contacts at the photosensitive sites in the adsorbed molecules each other. Thus, the efficiency of the crosslinking also decreases in this case, and consequently, the resultant film shows a small crosslinking rate. Accordingly, n in the hydrocarbon group (CH2)n is preferable to be in the range of 1 to 20. Nevertheless, in terms of crosslinking performance, n is preferable to be in the range of 3 to 16, and more preferable to be in the range of 5 to 10.
In the aggregate of adsorbed molecules that constitutes the liquid crystal alignment film, the adsorbed molecules may be aligned in a predetermined direction. Thereby, a uniform liquid crystal alignment performance is obtained.
In the above-described alignment film, and an inclination and/or alignment direction of long axes of the adsorbed molecules with respect to a substrate surface plane may be different in a plurality of divided domains next to each other. Thereby, the problem of viewing angle dependency in liquid crystal displays is resolved since the transmitted light in each pixel is made up of a plurality of bundles of lights.
In the above-described alignment film, the adsorbed molecules may be crosslinked with each other at the carbon Cxe2x80x2 and/or the carbon Cxe2x80x3 in the chemical formula 401 or 402. In the aggregate of adsorbed molecules, the inclination and alignment direction of the adsorbed molecules are not varied by external degrading factors such as heat or friction since the adsorbed molecules are firmly bonded each other by crosslinking. Accordingly, a highly reliable liquid crystal alignment film can be obtained.
In the above-described liquid crystal alignment film, a film thickness thereof may be from 0.5 nm to less than 10 nm. Within this thickness range, high alignment control efficiency relative to film thicknesses is achieved, and no hindrance to light transmission and electric field is caused. Thus, the utility of the film as a liquid crystal alignment film is further increased.
In the above-described liquid crystal alignment film, the liquid crystal alignment film may be a monomolecular thin film. When the film is a monomolecular thin film, the efficiency in liquid crystal alignment control remarkably increases since every individual adsorbed molecule can directly influence the alignment of liquid crystal molecules.
ii) Next, methods for producing liquid crystal alignment films in accordance with the fourth group of the invention are described below.
In one embodiment of the fourth group of the invention, there is provided a method for producing a liquid crystal alignment film comprising at least the steps of preparing a chemisorption solution by dissolving in a non-aqueous solvent a chemical adsorbate compound represented by the chemical formula 403:
where A is a divalent functional group and X is a halogen; and contacting the chemisorption solution with a substrate surface having a pixel electrode formed thereon to adsorb molecules of the chemical adsorbate compound in the chemisorption solution onto the substrate surface.
The above-described method may further comprise, after the step of drain-drying, irradiating the adsorbed molecules on the substrate surface with a polarized ultraviolet light so that the adsorbed molecules are crosslinked with each other at a carbon-carbon double bond shown in the chemical formula 403.
Further, the above-described method may be such that the steps of drain-drying and irradiating with a polarized ultraviolet light are repeated a plurality of times, the direction of the draining and drying is varied each time of the step of drain-drying, and one of a) a region to be irradiated with the ultraviolet light and a direction of the ultraviolet light, b) a region to be irradiated with the ultraviolet light and an incident angle of the ultraviolet light, and c) a region to be irradiated with the ultraviolet light, a direction of the ultraviolet light, and an angle of applying the ultraviolet light, is varied each time of the step of irradiating, whereby an inclination and/or alignment direction of long axes of the adsorbed molecules is/are varied in a plurality of domains such that a single pixel region is divided into the plurality of domains in a pattern-like manner. Accordingly, a multi-domain type liquid crystal alignment film can be efficiently fabricated with a high productivity.
Further in the above-described method, an aprotic solvent may be used for the nonaqueous solvent for cleaning in the step of rinsing, whereby unreacted chemical adsorbate compound is removed by rinsing the substrate surface to form a monomolecular thin film.
Further in the above-described method, a mixed solvent of an aprotic solvent and a protic solvent may be used for the nonaqueous solvent for cleaning in the step of rinsing, whereby unreacted chemical adsorbate compound is removed by rinsing the substrate surface to form a monomolecular thin film. The mixed solvent of an aprotic solvent and a protic solvent is preferable in that the capability of dissolving the chemical adsorbate compound and the evaporation rate can be appropriately controlled.
In the above-described method, the chemical adsorbate compound represented by the foregoing chemical formula 403 may be a compound represented by the following chemical formula 404.
(In the formula, n is an integer from 1 to 20, and X is a halogen.)
In the compound represented by the chemical formula 404, in which A in the chemical formula 403 is (CH2)nxe2x80x94Oxe2x80x94 (where n is an integer from 1 to 20), the photosensitivity of the carbon-carbon double bond is high. Moreover, the compound has a desirable form (being linear and having an appropriate molecular length) for controlling the alignment of liquid crystal molecules. Hence, the compound is suitable as a material for a liquid crystal alignment film. It is noted that more preferably, n should be from 5 to 10, and X should be chlorine in the chemical formula 404.
The significance of the above-described methods is now discussed below. When the substrate is contacted with the solution of the chemical adsorbate compound represented by the above chemical formula 403 (or the chemical formula 404), there is formed a thin film in which the chemical adsorbate compound is chemically adsorbed (normally, by siloxane bonds) onto hydrophilic groups on the substrate surface. The thin film is composed of an aggregate of adsorbed molecules aligned in such a manner that one longitudinal end (the end adjacent to the xe2x80x94Si bond group) of each adsorbed molecule is bonded to the substrate surface while the other end is extended towards a direction far from the substrate surface. In such a thin film, liquid crystal molecules can enter the gap spaces between the adsorbed molecules. The liquid crystal molecules slotted in the gap spaces are controlled by the alignment direction of the adsorbed molecules with respect to the substrate. Thus, by specifying the alignment direction of the adsorbed molecules, the alignment direction of the liquid crystal molecules can be controlled.
In the visible light range, the compound represented by the above chemical formula 403 or the chemical formula 404 is colorless, transparent, and chemically stable, while the carbon-carbon double bond thereof is highly sensitive to ultraviolet light. Accordingly, by irradiating the substrate with ultraviolet light, the adsorbed molecules can be crosslinked with each other via the carbon-carbon double bonds after the chemical adsorbate compound is chemically adsorbed onto the substrate. When a polarized ultraviolet light is used in this step, the direction of crosslinking can be controlled in a certain direction corresponding to the direction of polarization of the polarized ultraviolet light. By the polarized light irradiation, the adsorbed molecules on the substrate surface are realigned. By this realignment, the molecules each other are crosslinked, and as a result, the resultant alignment film is not easily changed by external degrading factors such as heat or friction.
The significance of the above-described methods is further detailed below. In the step of drain-drying of the above-described methods, first, unabsorbed chemical adsorbate compound excessively existing on the substrate surface is removed therefrom by rinsing, and thereafter, the solution for cleaning is removed by draining and drying. By these procedures, a monomolecular thin film can be formed such that the adsorbed molecules are aligned in a direction of drain-drying. The alignment state of the adsorbed molecules thus obtained by the drain-drying can be varied by repeating the drain-drying. Accordingly, the alignment state thus obtained is referred to as a xe2x80x9cpre-alignmentsxe2x80x9d herein.
It is noted that examples for the methods for the draining and drying include a method by pulling up the substrate being soaked in the cleaning solution at a predetermined angle, and a method by blowing an air current onto the substrate surface from a predetermined direction. In the above-described step of irradiating, the thin film surface (aggregate of the adsorbed molecules) is irradiated with a polarized ultraviolet light. Since the adsorbed molecules comprising the characteristic group represented by the chemical formula 403 are highly photosensitive, the molecules are reacted and crosslinked with each other at the carbon-carbon double bonds in a certain direction corresponding to the direction of polarization.
While the reason is not entirely understood, it is noted that, by pre-aligning the adsorbed molecules and thereafter irradiating with polarized ultraviolet light, the crosslinking is smoothly performed in a certain direction and the effect of the alignment treatment by the polarized ultraviolet irradiation is enhanced. The direction of polarization may be the same direction as the direction of the pre-alignment described above, or may be a different direction therefrom. In either case, by applying a polarized light, the adsorbed molecules can be realigned in a certain direction corresponding to the direction of polarization. It is, however, undesirable that the direction of drain-drying and the direction of polarization are crossed at 90xc2x0. This is because, if the directions are crossed at 90xc2x0, the molecules are randomly aligned in two directions. Therefore, it is preferable that the direction of drain-drying and the direction of polarization be staggered by several degrees or more.
It is to be noted that the alignment by the polarized ultraviolet light irradiation is referred to as a xe2x80x9crealignmentxe2x80x9d herein, in order to distinguish it from the pre-alignment described above. It is also to be noted that the compound molecules after being chemically adsorbed onto the substrate are referred to as xe2x80x9cadsorbed molecules,xe2x80x9d and the compound before being adsorbed are referred to as xe2x80x9cchemical adsorbate compound.xe2x80x9d
The differences between the liquid crystal alignment films according to the present invention and conventional liquid crystal alignment films are as follows. In the conventional liquid crystal alignment films (or example, polymer films composed of polyimides mentioned above), the long main chains are densely intertwined each other, and consequently, only the surface portion of the films can contribute to the alignment control over liquid crystals. For this reason, the conventional alignment films cannot achieve a sufficient alignment control performance. Moreover, in conventional alignment films in which the alignment control characteristic is obtained by rubbing, the alignment control characteristic is changed or degraded by external degrading factors such as hear or friction. Furthermore, polymer films composed of such as polyimides can hinder the light transmission and driving of liquid crystal since such films have a large film thickness and high electrical resistivity.
Nonetheless, even when the liquid crystal alignment film is made of a monomolecular thin film, the alignment stability can be insufficient if the adsorbed molecules are not crosslinked with each other. For example, the previously mentioned chemical adsorbate compound disclosed in Japanese Unexamined Patent Publication No. 3-7913 does not have a photoreactive group, and therefore is not capable of chemically linking the adsorbed molecules each other. Therefore, in the liquid crystal alignment films employing this chemical adsorbate compound, the alignment control characteristic tends to be degraded by heat at about 200xc2x0 C. It is noted that it has been confirmed that the thickness of the liquid crystal alignment film of the present invention, in which the adsorbed molecules are chemically adsorbed onto the substrate surface, is approximately the molecular length of the adsorbed molecule ( in the order of nanometers).
iii) Now, liquid crystal display devices in accordance with the fourth group of the invention utilizing the above-described liquid crystal alignment films are described below.
In one embodiment of the fourth group of the invention, there is provided a liquid crystal display device comprising a pair of opposed substrates, a liquid crystal alignment film provided on a surface of at least one of the substrates, the surface having an electrode thereon, and a liquid crystal enclosed in a cell gap between the pair of substrates, the liquid crystal display device wherein the liquid crystal alignment film comprises an aggregate of adsorbed molecules chemically adsorbed directly or via a layer of another substance onto the surface of the substrate having an electrode thereon, and each of the adsorbed molecules comprises an Si bond group at an end of the molecule and a characteristic group represented by the chemical formula 401:
In the above-described liquid crystal display device, each of the adsorbed molecules may be composed of a compound represented by the chemical formula 402:
(In the formula, n is an integer from 1 to 20.)
In the above-described liquid crystal display device, a pretilt angle and/or pretilt orientation of liquid crystal molecules in the cell gap may be controlled by an inclination and/or alignment direction of long axes of the adsorbed molecules with respect to a substrate surface plane.
In another embodiment of the fourth group, there is provided an in-plane switching type liquid crystal display device comprising; a substrate, a pixel electrode provided on the substrate, a counter electrode also provided on the substrate on which the pixel electrode is provided, and a liquid crystal alignment film formed on a surface of the substrates on which both of the electrodes are provided, the liquid crystal display device wherein; the liquid crystal alignment film comprises an aggregate of adsorbed molecules chemically adsorbed directly or via a layer of another substance onto the surface of the substrate having the electrodes thereon, and each of the adsorbed molecules comprises an Si bond group at an end of the molecule and a characteristic group represented by the foregoing chemical formula 401. In addition, each of the adsorbed molecules may be composed of a compound represented by the foregoing chemical formula 402.
In the above-described liquid crystal display device (hereafter including the in-plane switching type where applicable), a pretilt angle and/or pretilt orientation of liquid crystal molecules in the cell gap may be controlled by an inclination and/or alignment direction of long axes of the adsorbed molecules with respect to a substrate surface plane.
In the above-described liquid crystal display device, the adsorbed molecules may be crosslinked with each other at the carbon Cxe2x80x2 and/or the carbon Cxe2x80x3 in the chemical formula 401 (or the chemical formula 402).
In the above-described liquid crystal display device, a film thickness of the liquid crystal alignment film may be from 0.5 nm to less than 10 nm.
In the above-described liquid crystal display device, the liquid crystal alignment film may be a monomolecular thin film. When the alignment film is a monomolecular ultra thin film, the film does not hinder the electric field for driving liquid crystal or the light transmission if disposed in the transmission light path. Accordingly, a liquid crystal display device having an excellent brightness and a reduced driving voltage can be achieved.
It is considered that in an ideal monomolecular layer, each single constituent molecule is arrayed along a substrate surface and no molecules are overlaid thereon. However, in practice, it is not so easy to form a perfect monomolecular layer, and moreover, a monomolecular layer that is not so perfect can sufficiently accomplish the objects of the invention. Accordingly, it is to be understood that a monomolecular thin film according to the present invention may be such a thin film that it is recognized as a substantially monomolecular layer. For example, it may be such a film partially having a layer of a plurality of molecules in which unadsorbed molecules are overlaid on the adsorbed molecules onto a substrate. Also included is such a film partially having a layer of a plurality of molecules in which molecules that are themselves unbonded to the substrate but bonded to the molecules already fixed to the substrate, or further other molecules are bonded to the unbonded molecules. It is to be understood that monomolecular thin films according to the present invention includes these films partially having a layer of a plurality of molecules.
In addition, although the above discussion has been made based on the premise that an aggregate of adsorbed molecules is composed only of one type of chemical adsorbate compound, it is to be understood that other adsorbate compounds and other additives may be mixed with the adsorbed molecules of the present invention.